The present invention relates to a series of new azetidinone derivatives which may be used as intermediates in the preparation of certain carbapenem antibiotics.
The present invention also relates to a new process for preparing a series of carbapenem and penem compounds which are useful as antibiotics. The invention also provides a series of novel intermediates used in this process.
The penam and cephem antibiotics have been known for many years and have proved of considerable value in the control and prevention of infectious diseases. More recently, penem and carbapenem antibiotics have been developed and have also been found to be of great benefit. The penem and carbapenem compounds have in common a basic structure which may be represented by the formula (A'): ##STR2## in which Z' represents a sulfur atom or a group of formula &gt;CH.sub.2, which may optionally be substituted by an alkyl or alkoxy group. Those compounds in which Z' represents a sulfur atom are the penem compounds, whilst those in which Z' represents a group of formula &gt;CH.sub.2 or substituted group of formula &gt;CH.sub.2 are the carbapenem compounds. In accordance with the recommendations of the International Union of Pure and Applied Chemistry (IUPAC), the compounds referred to herein are named semi-systematically using penem and carbapenem as the parent compound. For the avoidance of doubt, the above formula (A') also shows the relevant numbers in the peripheral numbering system employed to describe the compounds herein.
Those carbapenem antibiotics having no substituent at the I-position are potentially a very useful series of compounds which have extraordinarily potent antibacterial activity. Unfortunately, however, they are chemically unstable and, moreover, are sensitive to dehydropeptidase I in vivo. Dehydropeptidase I is an enzyme which hydrolyses the .beta.-lactam ring in carbapenem antibiotics and which exists in mammalian tissue, for example in the renal cortex. It is responsible for the extensive metabolisation of many otherwise valuable .beta.-lactam antibiotics in animals, including humans, thus greatly reducing their value. Despite these disadvantages, these carbapenem antibiotics are finding increasing use in the treatment of bacterial infections. On the other hand, those carbapenem antibiotics having a 1.beta.-substituent are chemically stable and are resistant to the dehydropeptidase I enzyme. However, none of this series of compounds has been found in nature, and the compounds must, accordingly, be prepared by chemical synthesis. As with many biologically active compounds, the steric configuration of some of the atoms in the molecules of these compounds is of importance and the most interesting compounds have a multi-ring structure whose skeleton may be represented by the formula (B): ##STR3## in which each of the symbols R represents any of a variety of substituent groups, some of which may be quite complex, and the different groups represented by R in this formula may be the same or different, although they are normally different from each other. The numbering system shown on this formula is that commonly used in the art for the nomenclature of such compounds and is as used herein.
In the preparation of these compounds, it is necessary to synthesise an azetidinone ring system with various substituent groups preferably in the desired final configuration. This, in general, has proven difficult, although many attempts have been made. For example, U.S. Pat. Nos. 4,895,939 and 4,772,683 describe the preparation of a compound of formula (C): ##STR4## (in which tBu represents a t-butyl group and Me represents a methyl group) by reacting a compound of formula (D): ##STR5## with t-butyldimethylsilyl trifluoroacetate, which has the formula CF.sub.3 COOSi(CH.sub.3).sub.2 tBu, in the presence of a base to give a 75:25 mixture of compounds of formula (E) and (F): ##STR6## This mixture is then reacted with a compound of formula (G): ##STR7## in the presence of a Lewis acid, to give the desired compound of formula (C).
It has been reported that a 2R-isomer of a compound of formula (H): ##STR8## which is a key intermediate in the synthesis of 1.beta.-methylcarbapenem antibiotics, can be synthesized by reacting a silyl enol ether prepared from S-phenyl thiopropionate with (3R,4R)-3-[(1R)-1-t-butyldimethylsilyloxy)ethyl]-4-acetoxy-2-azetidinone or with its 1-trimethylsilyl derivative [T. Shibata et al., Tetrahedron Letters, 26, 4793 (1985); C. U. Kim et al., Tetrahedron Letters, 28, 507 (1987); A. Martel et al., Can. J. Chem., 66, 1537 (1988)].
However, in the syntheses described in these reports, the 2R- and 2S-isomers of the thiopropionic acid derivative of formula (H) are produced in the ratio of 1.6:1, 1:19 and 1:9, respectively. Thus the desired 2R-isomer is prepared in relatively minor amounts and, in most cases, is produced in admixture with a much larger quantity of its less useful 2S-isomer, or, at least, with a substantial quantity of the 2S-ismoer, from which its separation is difficult, expensive and inefficient.
There is, therefore, a need for a method of preparing the desired carbapenem antibiotic precursors which allows the required compounds to be obtained in better yields and with the desired isomer as the major product and not in admixture with substantial amounts of an unwanted isomer.
Several compounds having a thiol group attached to the 2-position of a penem or carbapenem compound are known and are thought to be of value as antibiotics. Many such compounds are prepared synthetically. In particular, the 1.beta.-methylcarbapenem derivatives, which are currently of considerable interest in this field, must be prepared synthetically, as no microorganism has been found which secretes them. An excellent method of synthesizing such compounds, which overcomes many of the disadvantages of the prior processes, is described in Japanese Patent Kokai Application No. Hei 1-25780 and involves oxidizing a sulfur atom at the carbapenem 2-position to an S-oxide (i.e. a sulfinyl or sulfonyl group) and then replacing the resulting sulfinyl or sulfonyl group by a desired mercapto group, for example as shown in the following reaction: ##STR9##
However, when this reaction is carried out by conventional means in which an organic base is employed, in many cases the sulfenic acid formed by the substitution reaction induces the production of various kinds of by-products, which results in a low reaction yield. For example, the yield of the addition-elimination reaction shown in the above reaction scheme is said to be 22% in Japanese Patent Kokai Application No. Hei 1-25780.
We have now surprisingly found that, if the reaction is effected in the presence of a metal salt of a metal of Group II or III of the Periodic Table of the Elements, in place of the organic base, yields can be much improved and yields of 40 to 80% or more (sometimes over 90%) can be achieved, instead of the yields of, at best, around 20% achievable in the prior art.